ABSTRACT: Circular RNAs (circRNAs) are an emerging class of RNA molecules with potential for prolonged expression due to their inaccessibility to exonucleases. Recent studies dissecting circRNA biogenesis have found inverted repeat sequences such as ALU repeats in humans, flanking a large number of exons that are subject to circularization and have shown these cis-elements as well as splicing machinery are essential for their circularization. In the mammalian brain, circRNAs appear to be highly abundant and dynamically regulated by development and plasticity. In particular, they appear to be enriched at the synapses and during neuronal differentiation and development. Despite these exciting advances, no systems for studying biogenesis of synthetic circRNAs in the brain and models of neurological disease. Specific aims for the current proposal are focused on engineering circRNA expression systems, studying factors affecting circRNA biogenesis efficiency in the brain, and utilizing these systems to interrogate deregulation of splicing and RNA-induced neurotoxicity seen in a C9orf72 Hexanucleotide Repeat Expansion mouse model of Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). CircRNAs capable of disrupting the neurotoxic RNA foci seen in these mouse models may enable design of therapeutics for FTD/ALS, where the C9orf72 expansion is prevalent and believed to be causative. Overall, the current proposal will help develop new strategies for efficient biogenesis of circRNAs in animal models, which are likely to help understand factors affecting circRNA biogenesis in the mammalian brain as well as usher in a new class of nucleic acid therapeutics based on circular RNAs.